1. Field of the Invention
The present invention relates generally to Ethernet passive optical networks, and more particularly, to a local area network emulation method and apparatus using logical link identifiers that are used for point-to-point communications and broadcasting in an Ethernet passive optical network.
2. Description of the Related Art
Currently, for the economical construction of a network suitable for the provision of broadband services, a Passive Optical Network (PON) based on Ethernet attracts attention. Ethernet PON (EPON) has been standardized by the IEEE 802.3ah Ethernet First Mile (EFM) task force. In general, PON is constructed to use Optical Distribution Networks (ODNs) between Optical Line Terminal (OLT) and Fiber To The Home (FTTH) Optical Network Terminations (ONTs) or Fiber To The Curb/Cabinet (FTTC) Optical Network Units (ONUs), and adapts a network topology in which all ONUs/ONTs are distributed in the form of a bus or tree structure.
FIG. 1 is a configuration diagram of an EPON system.
Referring to FIG. 1, the EPON system has a point-to-multipoint structure, in which a plurality of ONUs 101 installed on subscriber sides, such as houses or offices, share an Optical Line Terminal (OLT) 103 installed on a central office side and connected to the ONUs 101 through ODNs 102 using optical cables. Accordingly, downstream transmission in which a message is transmitted from the OLT 103 to the plurality of ONU 101 is performed in a broadcasting manner. In contrast, upstream transmission in which a message is transmitted from one of the plurality of ONUs 101 to the OLT 103 is performed in multipoint-to-point manner. In the EPON system constructed as described above, subscribers are provided with a variety of services, such as Internet services, telephone services and interactive video services, through video/audio networks, Internet Protocol (IP) networks, Asynchronous Transfer Mode (ATM) networks, Time Division Multiplexing (TDM)/Public Switched Telephone Networks (PSTN) networks, etc.
In such a PON scheme, it is possible for users to share exchange equipment and optical resources. The optical distribution network is passive, so that maintenance costs are low and high flexibility is provided to allow a provider to easily add and delete splits. Furthermore, the PON scheme is advantageous in that costs per subscriber are low because intensive resource sharing is realized.
FIG. 2 is a diagram showing the layered architectures of the OLT 103 and ONU 101 in a conventional EPON. The layered architectures shown in FIG. 2 have been adapted in IEEE802.3ah standards.
Referring to FIG. 2, the physical layers of the Open Systems Interconnection (OSI) reference model layers of the OLT 103 and the ONU 101 are constructed as described below. The OLT 103 and the ONU 101 are connected to a passive optical medium 210 through two Medium Dependent Interfaces (MDI) 211 and 241, respectively. Two Physical Medium Dependents (PMDs) 212 and 242, two Physical Medium Attachments (PMAs) 213 and 243, and two Physical Coding Sublayers (PSCs) 214 and 244 are connected to two reconciliation sublayers 216 and 246 through two Gigabit Media Independent Interfaces (GMII) 215 and 245, respectively. The reconciliation sublayers 216 and 246 are connected to two higher layers, that is, two data link layers, respectively.
Each of the two data link layers is divided into one or more Medium Access Control (MAC) layers 220 or 250, a multi-point MAC control layer 221 or 251, one or more optionally added Operations, Administration & Maintenance (OAM) layers 222 or 252, and one or more Logical Link Control (LLC) layers 223 or 253. The LLC layers 223 or 253 are connected to an upper layer 230 or 260.
In this case, the ONU 101 has a single MAC layer 250. In contrast, a plurality of ONUs 101 are connected to the OLT 103, so that the OLT 103 should accommodate independent MACs 220 the number of which corresponds to the number of connected ONUs 101. When the branching ratio between the OLT 103 and the ONU 101 is 1:16, 1:32 or 1:64, the OLT 103 has 16, 32 or 64 MACs 220 according to the corresponding branching ratio.
In general, the term “bridging” refers to a function of enabling data to be transmitted between networks by interconnecting the networks employing similar MAC protocols. Such bridging function is performed by the LLC layers 223 and 253 and the upper layers 230 and 260.
In the conventional layered architectures, the OLT 103 has independent MAC layers 220 and LLC layers 223, the number of each of which corresponds to the number of ONUs 101 connected to the OLT 103. In this case, a plurality of physical connection ports are required to connect the plurality of MAC layers 220 with the plurality of LLC layers 223. However, a problem arises in that it is very difficult to form and match the plurality of independent physical ports.